EP2707513A1 - Method for the production of very-high-strength martensitic steel and sheet or part thus obtained - Google Patents

Method for the production of very-high-strength martensitic steel and sheet or part thus obtained

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Publication number
EP2707513A1
EP2707513A1 EP12724656.9A EP12724656A EP2707513A1 EP 2707513 A1 EP2707513 A1 EP 2707513A1 EP 12724656 A EP12724656 A EP 12724656A EP 2707513 A1 EP2707513 A1 EP 2707513A1
Authority
EP
European Patent Office
Prior art keywords
steel
sheet
temperature
average
blank
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP12724656.9A
Other languages
German (de)
French (fr)
Other versions
EP2707513B1 (en
Inventor
Kangying ZHU
Olivier Bouaziz
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ArcelorMittal SA
Original Assignee
ArcelorMittal Investigacion y Desarrollo SL
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by ArcelorMittal Investigacion y Desarrollo SL filed Critical ArcelorMittal Investigacion y Desarrollo SL
Priority to PL12724656T priority Critical patent/PL2707513T3/en
Publication of EP2707513A1 publication Critical patent/EP2707513A1/en
Application granted granted Critical
Publication of EP2707513B1 publication Critical patent/EP2707513B1/en
Active legal-status Critical Current
Anticipated expiration legal-status Critical

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0231—Warm rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the invention relates to a process for the manufacture of sheets or parts made of steel with a martensitic structure, with a mechanical strength greater than that which could be obtained by austenitization, and then to a method for manufacturing steel sheets or parts made of steel with a martensitic structure.
  • simple fast cooling treatment with martensitic quenching, and strength and elongation properties for their application to the manufacture of energy absorbing parts in motor vehicles.
  • it is sought to produce steel parts combining high mechanical strength, high impact resistance and good corrosion resistance.
  • This type of combination is particularly desirable in the automotive industry where significant vehicle lightening is sought. This can be achieved particularly through the use of steel parts with very high mechanical properties whose microstructure is martensitic or bainito-martensitic.
  • Anti-intrusion parts, structure or participating in the safety of motor vehicles such as: bumper cross members, door or center pusher reinforcements, wheel arms, require for example the qualities mentioned above.
  • Their thickness is preferably less than 3 millimeters.
  • the patent EP0971044 thus discloses the manufacture of a steel sheet coated with aluminum or an aluminum alloy, the composition of which comprises in weight content: 0.15-0.5% C, 0.5- 3% Mn, 0.1-0.5% Si, 0.011% Cr, Ti ⁇ 0.2%, Al ⁇ 0.1%, P ⁇ 0.1%, S ⁇ 0.05%, 0.0005% ⁇ B ⁇ 0.08%, the balance being iron and impurities inherent in the elaboration.
  • This sheet is heated so as to obtain an austenitic transformation and hot stamped so as to produce a part, which is then cooled rapidly to obtain a martensitic or martensitobasitic structure. In this way, it is possible to obtain, for example, a mechanical strength greater than 1500 MPa. However, we seek to obtain parts with even greater mechanical strength. We search still, at a given level of mechanical strength, to reduce the carbon content of the steel so as to improve its weldability.
  • GB 1, 166,042 discloses a steel composition adapted to this process of ausforming, which comprises 0.1-0.6% C, 0.25-5% Mn, 0.5-2% AI , 0.5-3% Mo, 0.01 -2% Si, 0.01-1% V.
  • These steels include significant additions of molybdenum, manganese, aluminum, silicon and / or copper. These are intended to create a larger metastability domain for austenite, ie to delay the onset of the transformation from austenite to ferrite, bainite or perlite, at the temperature at which performs hot deformation.
  • Most studies on ausforming have been conducted on steels with a carbon content greater than 0.3%.
  • these compositions adapted to the ausforming have the disadvantage of requiring special precautions for welding, and also have particular difficulties in the case where it is desired to perform a metal coating quenching.
  • these compositions have expensive addition elements.
  • (C) denotes the carbon content of the steel, expressed as a percentage by weight.
  • a method of manufacture is thus sought which makes it possible to obtain an ultimate tensile strength of 50 MPa for expression (1), ie a strength greater than 3220 (C ) + 958 MPa for this steel. It seeks to have a method for the manufacture of sheet with a very high yield strength, that is greater than 1300 MPa. It is also sought to have a method for the manufacture of sheets or parts usable directly, that is to say without the need for a tempering treatment after quenching. It is also sought to have a manufacturing process for the manufacture of a sheet or a readily coated part by dipping in a metal bath.
  • the present invention aims to solve the problems mentioned above. It aims in particular to provide sheets with a yield strength greater than 1300 MPa, a mechanical strength expressed in megapascals greater than (3220 (C) +958) MPa, and preferably a total elongation greater than 3%.
  • the subject of the invention is a method for manufacturing a sheet of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5 , it being understood that the elongation factor of a
  • a semi-finished steel product whose composition comprises, the contents being expressed by weight, 0.15% ⁇ C ⁇ 0.40%, 1.5% ⁇ Mn ⁇ 3%, 0.005% ⁇ If ⁇ 2%, 0.005% ⁇ Al ⁇ 0.1%, 1, 8% ⁇ Cr ⁇ 4%, 0% ⁇ Mo ⁇ 2%, with 2.7% ⁇ 0.5 (Mn) + (Cr) +3 (Mo) ⁇ 5.7%, S ⁇ 0.05%, P ⁇ 0.1%, and optionally: 0% ⁇ Nb ⁇ 0.050%, 0.01% ⁇ Ti ⁇ 0.1 %, 0.0005% ⁇ B ⁇ 0.005%, 0.0005% ⁇ Ca ⁇ 0.005%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation,
  • the half-product is heated to a temperature Ti of between 1050 ° C. and 1250 ° C., and then
  • a rough rolling is carried out of the heated half-product at a temperature T 2 of between 1000 and 880 ° C., with a cumulative reduction rate ⁇ 3 of greater than 30% so as to obtain a sheet with a completely recrystallized austenitic structure; of average grain size less than 40 microns and preferably 5 microns, the cumulative reduction rate ⁇ 3 being defined by: Ln-,. e, a designating
  • the sheet is cooled to a temperature T3 of between 600 ° C. and 400 ° C. in the austenitic metastable domain, at a speed VRI greater than 2 ° C./s, and then
  • a finishing hot rolling is carried out at the temperature T 3 of the non-completely cooled sheet, with a cumulative reduction ratio b greater than 30% so as to obtain a sheet, the cumulative reduction ratio
  • the sheet is cooled at a speed V R2 greater than the critical speed of martensitic quenching.
  • the subject of the invention is also a process for manufacturing a piece of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of slats being between 2 and 5, comprising the successive steps and in this order according to which:
  • a steel blank whose composition comprises, the contents being expressed by weight, 0.15% ⁇ C ⁇ 0.40%, 1, 5% ⁇ Mn ⁇ 3%, 0.005% ⁇ Si ⁇ 2%, 0.005% ⁇ Al ⁇ 0.1%, 1, 8% ⁇ Cr ⁇ 4%, 0% ⁇ Mo ⁇ 2%, with 2.7% ⁇ 0.5 (Mn) + (Cr) +3 (Mo ) ⁇ 5.7%, S ⁇ 0.05%, P ⁇ 0.1%, optionally: 0% ⁇ Nb ⁇ 0.050%, 0.01% ⁇ Ti ⁇ 0.1%, 0.0005% ⁇ B ⁇ 0.005%, 0.0005% ⁇ Ca ⁇ 0.005%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation,
  • the blank is heated to a temperature Ti between Ac 3 and Ac 3 + 250 ° C. so that the average austenitic grain size is less than 40 microns, and preferably 5 microns, and then
  • the heated blank is transferred into a hot stamping press or a hot forming device, and then
  • the blank is cooled to a temperature T3 of between 600 ° C. and 400 ° C., at a speed V R greater than 2 ° C./s in order to avoid a transformation of the austenite,
  • the part is cooled to a speed V R2 greater than the critical speed of martensitic quenching.
  • the blank is hot-stamped so as to obtain a workpiece, then the workpiece is held in the stamping tool so as to cool it at a speed V R2 greater than the critical speed of martensitic quenching. .
  • the blank is pre-coated with aluminum or an aluminum-based alloy. According to another preferred embodiment, the blank is pre-coated with zinc or a zinc-based alloy.
  • the sheet or piece of steel obtained by any one of the above manufacturing processes is subjected to a subsequent heat treatment of tempering at a temperature T 4 of between 150 and 600 ° C. for a period of time between 5 and 30 minutes.
  • the subject of the invention is also an unreturned steel sheet having a yield strength greater than 1300 MPa, with a mechanical strength greater than (3220 (C) +958) megapascals, it being understood that (C) denotes the carbon content in weight percent of the steel, obtained according to any of the above manufacturing processes, of a totally martensitic structure, having an average slat size of less than 1 micrometer, the average elongation factor of slats being between 2 and 5
  • the invention also relates to a piece of unreturned steel obtained by any one of the above part manufacturing processes, the part comprising at least one zone of totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5, the yield strength in said zone being greater than 1300 MPa and the mechanical strength being greater than (3220 (C) +958) megapascals, it being understood that (C) refers to the percentage carbon content of the steel.
  • the subject of the invention is also a sheet or a piece of steel obtained by the process with the above treatment of income, the steel having a totally martensitic structure, having in at least one zone an average slat size of less than 1 , 2 micrometer, the average elongation factor of the slats being between 2 and 5.
  • FIG. 1 shows an example of microstructure of steel sheet manufactured by the method according to the invention.
  • FIG. 2 shows an example of microstructure of the same steel manufactured by a reference method, by heating in the austenitic domain and then by simple martensitic quenching.
  • FIG. 3 shows an exemplary piece of steel microstructure manufactured by the process according to the invention.
  • the carbon content of the steel is less than 0.15% by weight, the quenchability of the steel is insufficient given the process used and it is not possible to obtain a totally martensitic structure.
  • this content is greater than 0.40%, welded joints made from these sheets or these parts have insufficient toughness.
  • the optimum carbon content for the implementation of the invention is between 0.16 and 0.28%.
  • Manganese lowers the initial formation temperature of martensite and slows the decomposition of austenite. In order to obtain sufficient effects to allow the implementation of the ausforming, the manganese content must not be less than 1, 5%. Moreover, when the manganese content exceeds 3%, segregated zones are present in excessive quantity which is detrimental to the implementation of the invention. A preferred range for the implementation of the invention is 8 to 2.5% Mn.
  • the silicon content must be greater than 0.005% so as to contribute to the deoxidation of the steel in the liquid phase.
  • the silicon should not exceed 2% by weight due to the formation of surface oxides which significantly reduce the processability in processes involving a continuous passage of the steel sheet in a coating metal bath.
  • Chromium and molybdenum are very effective in delaying the transformation of austenite and in separating the transformation domains Ferritic-pearlitic and bainitic, ferrito- pearlitic transformation occurring at temperatures above bainitic transformation. These transformation domains are in the form of two distinct "noses" in an isothermal transformation chart TTT (transformation-temperature-time) from the austenite, which allows the implementation of the method according to the invention.
  • TTT transformation-temperature-time
  • the chromium content of the steel must be between 1.8% and 4% by weight in order for its delay effect on the transformation of the austenite to be sufficient.
  • the chromium content of the steel takes into account the content of other elements that increase the quenchability such as manganese and molybdenum: in fact, given the respective effects of manganese, chromium and molybdenum on the transformations from the austenite, a combined addition of these elements must be carried out respecting the following condition, the respectively noted quantities (Mn) (Cr) (Mo) being expressed in weight percentage: 2.7% ⁇ 0.5 (Mn) + (Cr) 3 (MB) ⁇ 5.7%.
  • the molybdenum content must not exceed 2% because of its excessive cost.
  • the aluminum content of the steel according to the invention is not less than 0.005% so as to obtain sufficient deoxidation of the steel in the liquid state.
  • the aluminum content is greater than 0.1% by weight, casting problems may occur. It is also possible to form inclusions of alumina in too large quantities or sizes which play a detrimental role on toughness.
  • the sulfur and phosphorus contents of the steel are respectively limited to 0.05 and 0.1% in order to avoid a reduction in the ductility or toughness of the parts or sheets produced according to the invention.
  • the steel may optionally contain niobium and / or titanium, which makes it possible to refine further refinement of the grain. Due to the heat curing these additions confer, they must however be limited to 0.050% for niobium and between 0.01 and 0.1% for titanium so as not to increase the forces during hot rolling. .
  • the steel can also contain boron: indeed, the significant deformation of the austenite can accelerate the conversion to ferrite on cooling, a phenomenon that should be avoided. Addition of boron in an amount of between 0.0005 and 0.005% by weight makes it possible to guard against early ferritic transformation.
  • the steel can also contain calcium in an amount between 0.0005 and 0.005%: by combining with oxygen and sulfur, calcium prevents the formation of large inclusions, harmful for the ductility of the sheets or parts thus manufactured.
  • the rest of the composition of the steel consists of iron and unavoidable impurities resulting from the elaboration.
  • the sheets or steel parts manufactured according to the invention are characterized by a totally slab martensite structure of great fineness: due to the specific thermomechanical cycle and composition, the average size of the martensitic slats is less than 1 micrometer and their average elongation factor is between 2 and 5.
  • These microstructural characteristics are determined for example by observing the microstructure by scanning electron microscopy using a field effect gun ("MEB-FEG” technique) at a magnification higher than 1200x, coupled to an EBSD detector ("Electron Backscatter Diffraction"). It is defined that two contiguous slats are distinct when their disorientation is greater than 5 degrees.
  • the average slat size is defined by the intercepts method known per se: the mean size of the intercepted slats is evaluated by randomly defined lines with respect to the microstructure. The measurement is performed on at least 1000 martensitic slats in order to obtain a representative average value.
  • the morphology of individualized slats is determined by image analysis using software known in themselves: the maximum dimension L max and minimum ⁇ mm is determined for each lath martensite and? Max elongation factor. In order to be statistically representative, this
  • the method according to the invention makes it possible to manufacture either rolled sheets or hot-stamped or heat-formed parts. These two modes will be successively exposed.
  • the process for manufacturing hot-rolled sheets according to the invention comprises the following steps:
  • a semi-finished steel product the composition of which has been described above, is supplied.
  • This semi-finished product may for example be in the form of slab from continuous casting, thin slab or ingot.
  • a continuous casting slab has a thickness of about 200 mm, a thin slab a thickness of about 50-80 mm.
  • This semi-finished product is heated to a temperature Ti of between 1050 ° C. and 1250 ° C.
  • the temperature ⁇ is greater than A c3 , the total conversion temperature to austenite at heating.
  • This reheating thus makes it possible to obtain a complete austenitization of the steel as well as the dissolution of any possible niobium carbonitrides in the semi-finished product.
  • This reheating step also makes it possible to carry out the various subsequent hot rolling operations that will be presented: a roughing operation is carried out on the semi-finished product at a temperature T 2 of between 1000 and 880 ° C.
  • ⁇ 3 The cumulative reduction rate of the various stages of rolling at roughing is noted ⁇ 3 . If e, a is the thickness of the semi-finished product prior to hot rough rolling and ef is the thickness of the sheet after this e
  • the cumulative reduction ratio ⁇ 3 during rough rolling must be greater than 30%.
  • the austenite obtained is completely recrystallized with an average grain size of less than 40 micrometers or even 5 micrometers when the deformation ⁇ 3 is greater than 200% and when the temperature T 2 is between 950 and 880 ° C. .
  • the sheet is then not completely cooled, that is to say up to intermediate temperature T3, so as to avoid transformation of the austenite, at a speed V R greater than 2 ° C / s to a temperature T 3 of between 600 ° C and 400 ° C, temperature range in which Austenite is metastable, ie in a field where it should not be present under conditions of thermodynamic equilibrium.
  • Finishing is then carried out at the temperature T 3> the cumulative reduction ratio b being greater than 30%. Under these conditions, a plastically deformed austenitic structure is obtained in which recrystallization does not occur.
  • the sheet is then cooled at a speed VR 2 greater than the critical martensitic quenching speed.
  • the invention is not limited to this geometry and to this type of products, and can be implemented for the manufacture of long products, bars, profiles, by successive stages of hot deformation.
  • a steel blank whose composition contains by weight: 0.15% ⁇ C ⁇ 0.40%, 1.5% ⁇ Mn ⁇ 3%, 0.005% ⁇ Si ⁇ 2%, 0.005% ⁇ Al ⁇ 0, 1%, 1, 8% ⁇ Cr ⁇ 4%, 0% ⁇ Mo ⁇ 2%, with 2.7% ⁇ 0.5 (Mn) + (Cr) +3 (Mo) ⁇ 5 , 7%, S ⁇ 0.05%, P ⁇ 0.1%, and optionally: 0% ⁇ Nb ⁇ 0.050%, 0.01% ⁇ Ti ⁇ 0.1%, 0.0005% ⁇ B ⁇ 0.005% , 0.0005% ⁇ Ca ⁇ 0.005%.
  • This flat blank is obtained by cutting a sheet or a coil in a form related to the final geometry of the target part.
  • This blank may be uncoated or optionally pre-coated.
  • the pre-coating may be aluminum or an aluminum-based alloy.
  • the sheet may advantageously be obtained by continuously dipping in a bath of aluminum-silicon alloy comprising by weight 5-1 1% silicon, 2 to 4% iron, optionally between 5 and 30 ppm of calcium, the rest being aluminum and unavoidable impurities resulting from the elaboration.
  • the blank may also be pre-coated with zinc or a zinc-based alloy.
  • the pre-coating can be in particular of the type galvanized with continuous dipping (“Gl”) or galvanized-alloyed (“GA”)
  • the blank is heated to a temperature ⁇ between A c3 and A C 3 + 250 ° C.
  • the heating is preferably carried out in an oven under ordinary atmosphere; during this step, an alloying between the steel and the precoat is observed.
  • the alloyed coating protects the underlying steel from oxidation and decarburization and is suitable for subsequent hot deformation.
  • the blank is held at temperature ⁇ to ensure the homogeneity of the temperature within it. Depending on the thickness of the blank, for example from 0.5 to 3 mm, the holding time at the temperature ⁇ varies from 30 seconds to 5 minutes.
  • the steel structure of the blank is completely austenitic.
  • the limitation of the temperature to A C 3 + 250 ° C has the effect of restricting the magnification of the austenitic grain to an average size of less than 40 micrometers.
  • the average grain size is preferably less than 5 micrometers.
  • the blank thus heated is transferred into a hot stamping press or into a hot forming device: the latter may for example be a "roll-forming" device in which the The blank is progressively deformed by hot forming into a series of rolls until the final geometry of the desired part is reached.
  • the transfer of the blank to the press or to the shaping device must be carried out quickly enough not to cause transformation of the austenite.
  • the blank is then cooled at a speed VRI greater than 2 ° C./s in order to avoid the transformation of the austenite, to a temperature T3 of between 600 ° C. and 400 ° C., a temperature range in which the Austenite is metastable.
  • the stamping press or hot forming device it is also possible to reverse the order of these last two steps, ie to first cool the blank with a speed VRI greater than 2 ° C./s, and then to transfer this blank to stamping press or hot forming device, so that it can be stamped or shaped hot as follows.
  • the blank is stamped or shaped at a temperature T3 of between 400 and 600 ° C., this hot deformation can be carried out in a single step or in several successive steps, as in the case of the roll-forming mentioned above. -above.
  • the stamping starts it possible to obtain a part whose shape is not developable.
  • the cumulative deformation s c must be greater than 30% so as to obtain a deformed non-recrystallized austenite.
  • ⁇ the equivalent deformation defined in each point of the piece by and ⁇ 2 are the
  • the hot forming mode is chosen such that the condition sc > 30% is satisfied at any point in the part formed.
  • the part After hot deformation, the part is cooled at a speed V 2 greater than the critical speed of martensitic quenching so as to obtain a totally martensitic structure.
  • this cooling can be achieved by holding the piece in the tooling with close contact with it.
  • This cooling by thermal conduction can be accelerated by cooling the stamping tool, for example through channels machined in the tool for the circulation of a refrigerant.
  • the hot stamping process of the invention therefore differs from the usual method of starting hot stamping as soon as the blank has been positioned in the press.
  • the flow limit of the steel is the lowest at high temperature and the forces required by the press are the lowest.
  • the method according to the invention consists in observing a waiting time so that the blank reaches a temperature range suitable for the ausforming, then hot stamping the blank at a significantly lower temperature than in the usual process.
  • the stamping force required by the press is slightly higher but the final structure obtained thinner than in the usual process leads to greater mechanical properties of yield strength, strength and stability. ductility. To meet a specification corresponding to a given level of stress, it is therefore possible to reduce the thickness of the blanks and thereby reduce the stamping force of the parts according to the invention.
  • the hot deformation immediately after stamping must be limited, this high temperature deformation tending to favor the formation of ferrite in the most deformed areas, which is sought to avoid.
  • the method according to the invention does not include this limitation.
  • the sheets or the steel parts may be used as such or subjected to a heat treatment of tempering, carried out at a temperature T 4 of between 150 and 600 ° C. for a period of time. between 5 and 30 minutes.
  • This treatment of income has the effect of increasing the ductility at the price of a decrease in yield strength and strength.
  • the inventors have however demonstrated that the method according to the invention, which gives a tensile strength Rm of at least 50 MPa higher than that obtained after conventional quenching, retained this advantage, even after tempering with temperatures ranging from 150 to 600 ° C.
  • the fineness characteristics of the microstructure are preserved by this treatment of income, the average size of slats being less than 1, 2 micrometer, the average elongation factor of slats being between 2 and 5.
  • the yield strength Re the tensile strength Rm, and the total elongation A have been determined for sheets obtained by these different modes of manufacture.
  • microstructure of the plates obtained by Scanning Electron Microscopy was also observed by means of a field effect gun ("MEB-FEG” technique) and EBSD detector and quantified the average size of the slats of the martensitic structure and their factor. extension
  • Tests A1 and A2 designate tests carried out on the composition of steel A under two different conditions, the test B1 was made from the composition of steel B.
  • Figure 1 shows the microstructure obtained in the case of test A1.
  • Figure 2 shows the microstructure of the same steel simply heated to 1250 ° C, maintained for 30 minutes at this temperature and then quenched with water (A2 test)
  • the method according to the invention makes it possible to obtain a martensite with a average size of slats much thinner and less elongated than in the reference structure.
  • the AR values are 353 and of 306 MPa respectively.
  • the method according to the invention therefore makes it possible to obtain mechanical strength values that are clearly higher than those which would be obtained by a simple martensitic quenching.
  • This increase in strength (353 or 306 MPa) is equivalent to that which would be obtained from equation (1) by simple martensitic quenching applied to steels in which an additional addition of 0.11% or 0.09 about% would have been achieved.
  • Such an increase in the carbon content would, however, have adverse consequences with respect to the weldability and toughness, whereas the method according to the invention makes it possible to achieve very high values of mechanical strength without these disadvantages.
  • the plates produced according to the invention because of their lower carbon content, have good weldability by the usual processes, in particular spot resistance welding.
  • test B3 cooled to 50 ° C / s to 525 ° C, then cooled to above the critical martensitic quenching rate (test B3)
  • FIG. 3 shows the microstructure obtained in the B3 condition according to the invention, characterized by a very fine slat size (0.9 micrometres) and a low elongation factor.
  • the invention allows the manufacture of sheets, or bare or coated parts, with very high mechanical characteristics, under very satisfactory economic conditions.

Abstract

The invention relates to a method for the production of a steel sheet having a fully martensitic structure with an average lathe size of less than 1 micrometre, the average elongation factor of the lathes being between 2 and 5, wherein the elongation factor of a lathe of maximum dimension lmax and minimum dimension Imin is defined as Imax / Imin, with a yield point greater than 1300 MPa, and mechanical strength greater than (3220(C)+958) megapascals, (C) denoting the carbon weight content of the steel. The method comprises the following steps consisting in: supplying a semi-finished steel product having a composition containing, expressed as weight, 0.15% ≤ C ≤ 0.40%, 1,5% ≤ Mn ≤ 3%, 0.005% ≤ Si ≤ 2%, 0.005% ≤ Al ≤ 0.1 %, 1.8% ≤ Cr ≤ 4%, 0% ≤ Mo ≤ 2%, wherein 2.7% ≤ 0.5 (Mn)+(Cr)+3(Mo) ≤ 5.7%, S ≤ 0.05%, P ≤ 0.1 % and, optionally, 0% ≤ Nb ≤ 0.050%, 0.01 % ≤ Ti ≤ 0.1 %, 0.0005% ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, the remainder of the composition being formed by iron and the inevitable impurities resulting from production; heating the semi-finished product to a temperature T1 between 1050°C and 1250°C and, subsequently, subjecting the heated semi-finished product to rough rolling at a temperature T2 between 1000 and 880°C, with a cumulative reduction rate εa greater than 30%, such as to obtain a sheet having an austenitic structure that is totally recrystallised, with an average grain size of less than 40 micrometres and preferably less than 5 micrometres; and partially cooling the sheet, such as to prevent the transformation of the austenite, at a rate VR1 greater than 2°C/s to a temperature T3 between 600°C and 400°C in the metastable austenitic range, and, subsequently, subjecting the not completely cooled sheet to final hot rolling at temperature T3, with a cumulative reduction rate εb greater than 30%, such as to obtain a sheet that is cooled at a rate VR2 above the critical cooling rate.

Description

PROCEDE DE FABRICATION D'ACIER MARTENSITIQUE A TRES HAUTE RESISTANCE ET TÔLE OU PIECE AINSI OBTENUE L'invention concerne un procédé de fabrication de tôles ou de pièces en acier à structure martensitique, avec une résistance mécanique supérieure à celle qui pourrait être obtenue par austénitisation puis simple traitement de refroidissement rapide avec trempe martensitique, et des propriétés de résistance mécanique et d'allongement permettant leur application à la fabrication de pièces à absorption d'énergie dans les véhicules automobiles. Dans certaines applications, on cherche à réaliser des pièces en acier combinant une résistance mécanique élevée, une grande résistance aux chocs et une bonne tenue à la corrosion. Ce type de combinaison est particulièrement désirable dans l'industrie automobile où l'on recherche un allégement significatif des véhicules. Ceci peut être notamment obtenu grâce à l'utilisation de pièces d'aciers à très hautes caractéristiques mécaniques dont la microstructure est martensitique ou bainito-martensitique. Des pièces anti-intrusion, de structure ou participant à la sécurité des véhicules automobiles telles que : traverses de pare-choc, renforts de portière ou de pied milieu, bras de roue, nécessitent par exemple les qualités mentionnées ci-dessus. Leur épaisseur est préférablement inférieure à 3 millimètres.  The invention relates to a process for the manufacture of sheets or parts made of steel with a martensitic structure, with a mechanical strength greater than that which could be obtained by austenitization, and then to a method for manufacturing steel sheets or parts made of steel with a martensitic structure. simple fast cooling treatment with martensitic quenching, and strength and elongation properties for their application to the manufacture of energy absorbing parts in motor vehicles. In some applications, it is sought to produce steel parts combining high mechanical strength, high impact resistance and good corrosion resistance. This type of combination is particularly desirable in the automotive industry where significant vehicle lightening is sought. This can be achieved particularly through the use of steel parts with very high mechanical properties whose microstructure is martensitic or bainito-martensitic. Anti-intrusion parts, structure or participating in the safety of motor vehicles such as: bumper cross members, door or center pusher reinforcements, wheel arms, require for example the qualities mentioned above. Their thickness is preferably less than 3 millimeters.
Le brevet EP0971044 divulgue ainsi la fabrication d'une une tôle d'acier revêtue d'aluminium ou d'un alliage d'aluminium, dont la composition comprend en teneur pondérale : 0,15-0,5%C, 0,5-3%Mn, 0,1-0,5%Si, 0,011%Cr, Ti<0,2%, Al<0,1%, P<0,1%, S<0,05%, 0,0005%<B<0,08%, le reste étant du fer et des impuretés inhérentes à l'élaboration. Cette tôle est chauffée de façon à obtenir une transformation austénitique puis emboutie à chaud de manière à réaliser une pièce, celle-ci étant ensuite refroidie rapidement de façon à obtenir une structure martensitique ou martensito- bainitique. De la sorte, on peut obtenir par exemple une résistance mécanique supérieure à 1500MPa. On cherche cependant à obtenir des pièces avec une résistance mécanique encore supérieure. On cherche encore, à niveau donné de résistance mécanique, à diminuer la teneur en carbone de l'acier de façon à améliorer son aptitude à la soudabilité. The patent EP0971044 thus discloses the manufacture of a steel sheet coated with aluminum or an aluminum alloy, the composition of which comprises in weight content: 0.15-0.5% C, 0.5- 3% Mn, 0.1-0.5% Si, 0.011% Cr, Ti <0.2%, Al <0.1%, P <0.1%, S <0.05%, 0.0005% <B <0.08%, the balance being iron and impurities inherent in the elaboration. This sheet is heated so as to obtain an austenitic transformation and hot stamped so as to produce a part, which is then cooled rapidly to obtain a martensitic or martensitobasitic structure. In this way, it is possible to obtain, for example, a mechanical strength greater than 1500 MPa. However, we seek to obtain parts with even greater mechanical strength. We search still, at a given level of mechanical strength, to reduce the carbon content of the steel so as to improve its weldability.
On connaît également un procédé de fabrication appelé « ausforming » dans lequel un acier est totalement austénitisé puis refroidi rapidement jusqu'à une température intermédiaire, généralement vers 700-400°C, gamme dans laquelle l'austénite est métastable. Cette austénite est déformée à chaud puis refroidie rapidement de façon à obtenir une structure totalement martensitique. Le brevet GB1 ,080,304 décrit ainsi la composition d'une tôle d'acier destinée à un tel procédé, qui comprend 0,15-1 %C, 0,25-3%Mn, 1- 2,5%Si, 0,5-3%Mo, 1-3%Cu, 0,2-1 %V. There is also known a manufacturing process called "ausforming" in which a steel is totally austenitized and then rapidly cooled to an intermediate temperature, generally around 700-400 ° C, in which range the austenite is metastable. This austenite is hot deformed and then rapidly cooled so as to obtain a totally martensitic structure. GB1, 080,304 thus describes the composition of a steel sheet intended for such a process, which comprises 0.15-1% C, 0.25-3% Mn, 1- 2.5% Si, 0, 5-3% Mo, 1-3% Cu, 0.2-1% V.
De même, le brevet GB 1 ,166,042 décrit une composition d'acier adaptée à ce procédé d'ausforming, qui comprend 0,1-0,6%C, 0,25-5%Mn, 0,5-2%AI, 0,5-3%Mo, 0,01 -2%Si, 0,01 -1%V.  Similarly, GB 1, 166,042 discloses a steel composition adapted to this process of ausforming, which comprises 0.1-0.6% C, 0.25-5% Mn, 0.5-2% AI , 0.5-3% Mo, 0.01 -2% Si, 0.01-1% V.
Ces aciers comportent des additions importantes de molybdène, de manganèse, d'aluminium, de silicium et/ou de cuivre. Celles-ci ont pour but de créer un domaine de métastabilité plus important pour l'austénite, c'est-à- dire de retarder le début de la transformation de l'austénite en ferrite, bainite ou perlite, à la température à laquelle on effectue la déformation à chaud. La plupart des études consacrées à l'ausforming ont été menées sur des aciers présentant une teneur en carbone supérieure à 0,3%. Ainsi, ces compositions adaptées à l'ausforming présentent l'inconvénient de nécessiter des précautions particulières pour le soudage, et présentent également des difficultés particulières dans le cas où l'on souhaite effectuer un revêtement métallique au trempé. De plus, ces compositions comportent des éléments d'addition coûteux.  These steels include significant additions of molybdenum, manganese, aluminum, silicon and / or copper. These are intended to create a larger metastability domain for austenite, ie to delay the onset of the transformation from austenite to ferrite, bainite or perlite, at the temperature at which performs hot deformation. Most studies on ausforming have been conducted on steels with a carbon content greater than 0.3%. Thus, these compositions adapted to the ausforming have the disadvantage of requiring special precautions for welding, and also have particular difficulties in the case where it is desired to perform a metal coating quenching. In addition, these compositions have expensive addition elements.
On cherche à disposer d'un procédé de fabrication de tôles ou de pièces d'acier ne présentant pas les inconvénients ci-dessus, dotées d'une résistance à la rupture supérieure de plus de 50 MPa à celle que l'on pourrait obtenir grâce à une austénitisation suivie d'une simple trempe martensitique de l'acier en question. Les inventeurs ont mis en évidence que, pour des teneurs en carbone allant de 0,15 à 0,40% en poids, la résistance à la rupture en traction Rm d'aciers fabriqués par austénitisation totale suivie d'une simple trempe martensitique, ne dépendait pratiquement que de la teneur en carbone et était reliée à celle-ci avec une très bonne précision, selon l'expression (1) : Rm (mégapascals) = 3220(C) + 908. It seeks to have a method of manufacturing sheets or steel parts does not have the above disadvantages, with a higher tensile strength of more than 50 MPa that could be obtained through austenitization followed by a simple martensitic quenching of the steel in question. The inventors have demonstrated that, for carbon contents ranging from 0.15 to 0.40% by weight, the tensile tensile strength Rm of steels manufactured by total austenitization followed by a simple martensitic quench, do not depended practically only on the content of carbon and was connected to it with very good accuracy, according to the expression (1): Rm (megapascals) = 3220 (C) + 908.
Dans cette expression, (C) désigne la teneur en carbone de l'acier exprimée en pourcentage pondéral. A teneur en carbone C donnée pour un acier, on cherche donc un procédé de fabrication permettant d'obtenir une résistance à la rupture supérieure de 50 MPa à l'expression (1), c'est à dire une résistance supérieure à 3220(C)+ 958 MPa pour cet acier. On cherche à disposer d'un procédé permettant la fabrication de tôle à très haute limite d'élasticité, c'est à dire supérieure à 1300 MPa. On cherche également à disposer d'un procédé permettant la fabrication de tôles ou de pièces utilisables directement, c'est à dire sans nécessité impérative d'un traitement de revenu après trempe. On cherche également à disposer d'un procédé de fabrication permettant la fabrication d'une tôle ou d'une pièce aisément revêtable au trempé dans un bain métallique. In this expression, (C) denotes the carbon content of the steel, expressed as a percentage by weight. At a carbon content C given for a steel, a method of manufacture is thus sought which makes it possible to obtain an ultimate tensile strength of 50 MPa for expression (1), ie a strength greater than 3220 (C ) + 958 MPa for this steel. It seeks to have a method for the manufacture of sheet with a very high yield strength, that is greater than 1300 MPa. It is also sought to have a method for the manufacture of sheets or parts usable directly, that is to say without the need for a tempering treatment after quenching. It is also sought to have a manufacturing process for the manufacture of a sheet or a readily coated part by dipping in a metal bath.
Ces tôles ou ces pièces doivent être soudables par les procédés usuels et ne pas comporter d'additions coûteuses d'éléments d'alliage. These sheets or these parts must be weldable by the usual methods and not have expensive additions of alloying elements.
La présente invention a pour but de résoudre les problèmes évoqués ci- dessus. Elle vise en particulier à mettre à disposition des tôles avec une limite d'élasticité supérieure à 1300 MPa, une résistance mécanique exprimée en mégapascals supérieure à (3220(C)+958) MPa, et de préférence un allongement total supérieur à 3%. The present invention aims to solve the problems mentioned above. It aims in particular to provide sheets with a yield strength greater than 1300 MPa, a mechanical strength expressed in megapascals greater than (3220 (C) +958) MPa, and preferably a total elongation greater than 3%.
Dans ce but, l'invention a pour objet un procédé de fabrication d'une tôle d'acier à structure totalement martensitique présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen des lattes étant compris entre 2 et 5, étant entendu que le facteur d'allongement d'une  For this purpose, the subject of the invention is a method for manufacturing a sheet of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5 , it being understood that the elongation factor of a
/ ÏTltlX  / ÏTltlX
latte de dimension maximale lmax et minimale lmin est défini par , à limite latte of maximum dimension l ma x and minimum l m i n is defined by, at limit
/ min  / min
d'élasticité supérieure à 1300 MPa, à résistance mécanique supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral de l'acier, comprenant les étapes successives et dans cet ordre selon lesquelles : of elasticity greater than 1300 MPa, with a mechanical strength greater than (3220 (C) +958) megapascals, it being understood that (C) denotes the carbon content as a weight percentage of the steel, comprising the successive stages and in this order whereby :
- on approvisionne un demi-produit d'acier dont la composition comprend, les teneurs étant exprimées en poids, 0,15% < C < 0,40%, 1 ,5%< Mn < 3%, 0,005% < Si < 2%, 0,005%< Al < 0,1 %, 1 ,8% < Cr< 4%, 0%< Mo <2%, étant entendu que 2,7%<0,5 (Mn)+(Cr)+3(Mo)<5,7%, S ≤ 0,05%, P< 0,1 %, et optionnellement: 0%< Nb<0,050%, 0,01 %≤ Ti<0,1 %, 0,0005% < B < 0,005%, 0,0005% < Ca < 0,005%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration, a semi-finished steel product is supplied whose composition comprises, the contents being expressed by weight, 0.15% <C <0.40%, 1.5% <Mn <3%, 0.005% <If <2%, 0.005% <Al <0.1%, 1, 8% <Cr <4%, 0% <Mo <2%, with 2.7% <0.5 (Mn) + (Cr) +3 (Mo) <5.7%, S ≤ 0.05%, P <0.1%, and optionally: 0% <Nb <0.050%, 0.01% ≤ Ti <0.1 %, 0.0005% <B <0.005%, 0.0005% <Ca <0.005%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation,
- on réchauffe le demi-produit à une température Ti comprise entre 1050°C et 1250°C, puis  the half-product is heated to a temperature Ti of between 1050 ° C. and 1250 ° C., and then
- on effectue un laminage de dégrossissage du demi-produit réchauffé, à une température T2 comprise entre 1000 et 880°C, avec un taux de réduction ε3 cumulé supérieur à 30% de façon à obtenir une tôle avec une structure austénitique complètement recristallisée de taille moyenne de grain inférieure à 40 micromètres et préférentiellement à 5 micromètres, le taux de réduction cumulé ε3 étant défini par : Ln— ,. e,a désignant a rough rolling is carried out of the heated half-product at a temperature T 2 of between 1000 and 880 ° C., with a cumulative reduction rate ε 3 of greater than 30% so as to obtain a sheet with a completely recrystallized austenitic structure; of average grain size less than 40 microns and preferably 5 microns, the cumulative reduction rate ε 3 being defined by: Ln-,. e, a designating
J a  J
l'épaisseur du demi-produit avant le laminage à chaud de dégrossissage et efa l'épaisseur de la tôle après le laminage de dégrossissage, puisthe thickness of the semi-finished product before the hot rolling of roughing and e f the thickness of the sheet after the rough rolling, and then
- on refroidit non complètement la tôle jusqu'à une température T3 comprise entre 600°C et 400°C dans le domaine austénitique métastable, à une vitesse VRI supérieure à 2°C/s, puis the sheet is cooled to a temperature T3 of between 600 ° C. and 400 ° C. in the austenitic metastable domain, at a speed VRI greater than 2 ° C./s, and then
- on effectue un laminage à chaud de finition à la température T3, de la tôle non complètement refroidie, avec un taux de réduction cumulé £b supérieur à 30% de façon à obtenir une tôle, le taux de réduction cumulé a finishing hot rolling is carried out at the temperature T 3 of the non-completely cooled sheet, with a cumulative reduction ratio b greater than 30% so as to obtain a sheet, the cumulative reduction ratio
eib e ib
8b étant défini par : Ln— , e,b désignant l'épaisseur de la tôle avant le  8b being defined by: Ln-, e, b denoting the thickness of the sheet before
ef b ef b
laminage à chaud de finition et efa l'épaisseur de la tôle après le laminage de finition, puis finishing hot rolling and e fa the thickness of the sheet after finishing rolling, and then
- on refroidit la tôle à une vitesse VR2 supérieure à la vitesse critique de trempe martensitique. the sheet is cooled at a speed V R2 greater than the critical speed of martensitic quenching.
L'invention a également pour objet un procédé de fabrication d'une pièce d'acier à structure totalement martensitique présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen des lattes étant compris entre 2 et 5, comprenant les étapes successives et dans cet ordre selon lesquelles : The subject of the invention is also a process for manufacturing a piece of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of slats being between 2 and 5, comprising the successive steps and in this order according to which:
- on approvisionne un flan d'acier dont la composition comprend, les teneurs étant exprimées en poids, 0,15% < C < 0,40%,1 ,5%< Mn < 3%, 0,005% < Si < 2%, 0,005%< Al < 0,1%, 1 ,8% < Cr< 4%, 0%< Mo <2%, étant entendu que 2,7%<0,5 (Mn)+(Cr)+3(Mo)<5,7%, S < 0,05%, P< 0,1%, optionnellement: 0%< Nb<0,050%, 0,01 %≤ Ti≤0,1%, 0,0005% < B < 0,005%, 0,0005% < Ca < 0,005%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration,  a steel blank whose composition comprises, the contents being expressed by weight, 0.15% <C <0.40%, 1, 5% <Mn <3%, 0.005% <Si <2%, 0.005% <Al <0.1%, 1, 8% <Cr <4%, 0% <Mo <2%, with 2.7% <0.5 (Mn) + (Cr) +3 (Mo ) <5.7%, S <0.05%, P <0.1%, optionally: 0% <Nb <0.050%, 0.01% ≤ Ti≤0.1%, 0.0005% <B < 0.005%, 0.0005% <Ca <0.005%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation,
- on chauffe le flan à une température Ti comprise entre Ac3 et Ac3+250°C de telle sorte que la taille moyenne de grain austénitique soit inférieure à 40 micromètres, et préférentiellement à 5 micromètres, puis the blank is heated to a temperature Ti between Ac 3 and Ac 3 + 250 ° C. so that the average austenitic grain size is less than 40 microns, and preferably 5 microns, and then
- on transfère le flan chauffé au sein d'une presse d'emboutissage à chaud ou d'un dispositif de mise en forme à chaud, puis  the heated blank is transferred into a hot stamping press or a hot forming device, and then
- -on refroidit le flan jusqu'à une température T3 comprise entre 600°C et 400°C, à une vitesse VR supérieure à 2°C/s de façon à éviter une transformation de l'austénite, the blank is cooled to a temperature T3 of between 600 ° C. and 400 ° C., at a speed V R greater than 2 ° C./s in order to avoid a transformation of the austenite,
- l'ordre des deux dernières étapes pouvant être interverti, puis,  the order of the last two steps that can be inverted, then
- on emboutit ou on met en forme à chaud à la température T3 le flan refroidi, d'une quantité sc supérieure à 30% dans au moins une zone, pour obtenir une pièce , sc étant défini par ec = = ](s + εχε2 + ε2 2 ) , où ει et ε2 the cooled blank, of an amount s c greater than 30% in at least one zone, is stamped or shaped at a temperature T 3 to obtain a part, s c being defined by e c = =] (s + ε χ ε 2 + ε 2 2 ), where ει and ε 2
V3  V3
sont les déformations principales cumulées sur l'ensemble des étapes de déformation à la température T3, puis, are the main deformations accumulated over all the deformation steps at the temperature T 3 , then,
- on refroidit la pièce à une vitesse VR2 supérieure à la vitesse critique de trempe martensitique. the part is cooled to a speed V R2 greater than the critical speed of martensitic quenching.
Selon un mode préféré, le flan est embouti à chaud de façon à obtenir une pièce, puis la pièce est maintenue au sein de l'outillage d'emboutissage de façon à la refroidir à une vitesse VR2 supérieure à la vitesse critique de trempe martensitique. In a preferred embodiment, the blank is hot-stamped so as to obtain a workpiece, then the workpiece is held in the stamping tool so as to cool it at a speed V R2 greater than the critical speed of martensitic quenching. .
Selon un mode préféré, le flan est pré-revêtu d'aluminium ou d'un alliage à base d'aluminium. Selon un autre mode préféré, le flan est pré-revêtu de zinc ou d'un alliage à base de zinc. In a preferred embodiment, the blank is pre-coated with aluminum or an aluminum-based alloy. According to another preferred embodiment, the blank is pre-coated with zinc or a zinc-based alloy.
Préférentiellement, la tôle ou la pièce d'acier obtenue par l'un quelconque des procédés de fabrication ci-dessus, est soumise à un traitement thermique ultérieur de revenu à une température T4 comprise entre 150 et 600°C pendant une durée comprise entre 5 et 30 minutes. Preferably, the sheet or piece of steel obtained by any one of the above manufacturing processes is subjected to a subsequent heat treatment of tempering at a temperature T 4 of between 150 and 600 ° C. for a period of time between 5 and 30 minutes.
L'invention a également pour objet une tôle d'acier non revenu de limite d'élasticité supérieure à 1300 MPa, de résistance mécanique supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral de l'acier, obtenue selon un quelconque des procédés de fabrication ci-dessus, de structure totalement martensitique, présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen des lattes étant compris entre 2 et 5  The subject of the invention is also an unreturned steel sheet having a yield strength greater than 1300 MPa, with a mechanical strength greater than (3220 (C) +958) megapascals, it being understood that (C) denotes the carbon content in weight percent of the steel, obtained according to any of the above manufacturing processes, of a totally martensitic structure, having an average slat size of less than 1 micrometer, the average elongation factor of slats being between 2 and 5
L'invention a également pour objet une pièce d'acier non revenu obtenue par l'un quelconque des procédés de fabrication de pièce ci-dessus, la pièce comportant au moins une zone de structure totalement martensitique présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen des lattes étant compris entre 2 et 5, la limite d'élasticité dans ladite zone étant supérieure à 1300 MPa et la résistance mécanique étant supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral de l'acier. L'invention a également pour objet une tôle ou une pièce d'acier obtenue par le procédé avec traitement de revenu ci-dessus, l'acier ayant une structure totalement martensitique, présentant dans au moins une zone une taille moyenne de lattes inférieure à 1,2 micromètre, le facteur d'allongement moyen des lattes étant compris entre 2 et 5. The invention also relates to a piece of unreturned steel obtained by any one of the above part manufacturing processes, the part comprising at least one zone of totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5, the yield strength in said zone being greater than 1300 MPa and the mechanical strength being greater than (3220 (C) +958) megapascals, it being understood that (C) refers to the percentage carbon content of the steel. The subject of the invention is also a sheet or a piece of steel obtained by the process with the above treatment of income, the steel having a totally martensitic structure, having in at least one zone an average slat size of less than 1 , 2 micrometer, the average elongation factor of the slats being between 2 and 5.
Les inventeurs ont mis en évidence que les problèmes exposés ci-dessus étaient résolus grâce à un procédé d'ausforming spécifique mis en œuvre sur une gamme particulière de compositions d'aciers. Contrairement aux études précédentes qui montraient que l'ausforming requérait l'addition d'éléments d'alliage coûteux, les inventeurs ont mis en évidence de façon surprenante que cet effet peut être obtenu grâce à des compositions nettement moins chargées en éléments d'alliage. D'autres caractéristiques et avantages de l'invention apparaîtront au cours de la description ci-dessous donnée à titre d'exemple et faite en référence aux figures jointes suivantes : The inventors have demonstrated that the problems described above were solved by means of a specific ausforming process implemented on a particular range of steel compositions. Contrary to previous studies which showed that ausforming required the addition of expensive alloying elements, the inventors have surprisingly demonstrated that this effect can be obtained thanks to substantially less charged compositions of alloying elements. Other features and advantages of the invention will become apparent from the following description given by way of example and with reference to the following appended figures:
La figure 1 présente un exemple de microstructure de tôle d'acier fabriquée par le procédé selon l'invention.  FIG. 1 shows an example of microstructure of steel sheet manufactured by the method according to the invention.
La figure 2 présente un exemple de microstructure du même acier fabriqué par un procédé de référence, par chauffage dans le domaine austénitique puis simple trempe martensitique.  FIG. 2 shows an example of microstructure of the same steel manufactured by a reference method, by heating in the austenitic domain and then by simple martensitic quenching.
La figure 3 présente un exemple de microstructure de pièce d'acier fabriquée par le procédé selon l'invention.  FIG. 3 shows an exemplary piece of steel microstructure manufactured by the process according to the invention.
La composition des aciers mis en œuvre dans le procédé selon l'invention va maintenant être détaillée.  The composition of the steels used in the process according to the invention will now be detailed.
Lorsque la teneur en carbone de l'acier est inférieure à 0,15% en poids, la trempabilité de l'acier est insuffisante compte tenu du procédé mis en œuvre et il n'est pas possible d'obtenir une structure totalement martensitique. Lorsque cette teneur est supérieure à 0,40%, les joints soudés réalisés à partir de ces tôles ou de ces pièces présentent une ténacité insuffisante. La teneur optimale en carbone pour la mise en œuvre de l'invention est comprise entre 0,16 et 0,28%.  When the carbon content of the steel is less than 0.15% by weight, the quenchability of the steel is insufficient given the process used and it is not possible to obtain a totally martensitic structure. When this content is greater than 0.40%, welded joints made from these sheets or these parts have insufficient toughness. The optimum carbon content for the implementation of the invention is between 0.16 and 0.28%.
Le manganèse abaisse la température de début de formation de la martensite et ralentit la décomposition de l'austénite. Afin d'obtenir des effets suffisants pour permettre la mise en œuvre de l'ausforming, la teneur en manganèse ne doit pas être inférieure à 1 ,5%. Par ailleurs, lorsque la teneur en manganèse dépasse 3%, des zones ségrégées sont présentes en quantité excessive ce qui nuit à la mise en œuvre de l'invention. Une gamme préférentielle pour la mise en œuvre de l'invention est ,8 à 2,5%Mn. Manganese lowers the initial formation temperature of martensite and slows the decomposition of austenite. In order to obtain sufficient effects to allow the implementation of the ausforming, the manganese content must not be less than 1, 5%. Moreover, when the manganese content exceeds 3%, segregated zones are present in excessive quantity which is detrimental to the implementation of the invention. A preferred range for the implementation of the invention is 8 to 2.5% Mn.
La teneur en silicium doit être supérieure à 0,005% de façon à contribuer à la désoxydation de l'acier en phase liquide. Le silicium ne doit pas excéder 2% en poids en raison de la formation d'oxydes superficiels qui réduisent notablement la revêtabilité dans les procédés comportant un passage en continu de la tôle d'acier dans un bain métallique de revêtement.  The silicon content must be greater than 0.005% so as to contribute to the deoxidation of the steel in the liquid phase. The silicon should not exceed 2% by weight due to the formation of surface oxides which significantly reduce the processability in processes involving a continuous passage of the steel sheet in a coating metal bath.
Le chrome et le molybdène sont des éléments très efficaces pour retarder la transformation de l'austénite et pour séparer les domaines de transformation ferrito-perlitique et bainitique, la transformation ferrito-perlitique intervenant à des températures supérieures à la transformation bainitique. Ces domaines de transformation se présentent sous forme de deux « nez » bien distincts dans un diagramme de transformation isotherme TTT (Transformation- Température-Temps) à partir de l'austénite, ce qui permet la mise en œuvre du procédé selon l'invention. Chromium and molybdenum are very effective in delaying the transformation of austenite and in separating the transformation domains Ferritic-pearlitic and bainitic, ferrito- pearlitic transformation occurring at temperatures above bainitic transformation. These transformation domains are in the form of two distinct "noses" in an isothermal transformation chart TTT (transformation-temperature-time) from the austenite, which allows the implementation of the method according to the invention.
La teneur en chrome de l'acier doit être comprise entre 1 ,8% et 4% en poids pour que son effet de retardement sur la transformation de l'austénite soit suffisant. La teneur en chrome de l'acier tient compte de la teneur d'autres éléments augmentant la trempabilité tels que la manganèse et le molybdène : en effet, compte tenu des effets respectifs du manganèse, du chrome et du molybdène sur les transformations à partir de l'austénite, une addition combinée de ces éléments doit être effectuée en respectant la condition suivante, les quantités respectivement notées (Mn) (Cr) (Mo) étant exprimées en pourcentage pondéral : 2,7%<0,5 (Mn)+(Cr)+3(Mo)<5,7%.  The chromium content of the steel must be between 1.8% and 4% by weight in order for its delay effect on the transformation of the austenite to be sufficient. The chromium content of the steel takes into account the content of other elements that increase the quenchability such as manganese and molybdenum: in fact, given the respective effects of manganese, chromium and molybdenum on the transformations from the austenite, a combined addition of these elements must be carried out respecting the following condition, the respectively noted quantities (Mn) (Cr) (Mo) being expressed in weight percentage: 2.7% <0.5 (Mn) + (Cr) 3 (MB) <5.7%.
La teneur en molybdène ne doit cependant pas excéder 2% en raison de son coût excessif.  However, the molybdenum content must not exceed 2% because of its excessive cost.
La teneur en aluminium de l'acier selon l'invention n'est pas inférieure à 0,005% de façon à obtenir une désoxydation suffisante de l'acier à l'état liquide. Lorsque la teneur en aluminium est supérieure à 0,1% en poids, des problèmes de coulée peuvent apparaître. Il peut également se former des inclusions d'alumine en quantité ou en taille trop importantes qui jouent un rôle néfaste sur la ténacité.  The aluminum content of the steel according to the invention is not less than 0.005% so as to obtain sufficient deoxidation of the steel in the liquid state. When the aluminum content is greater than 0.1% by weight, casting problems may occur. It is also possible to form inclusions of alumina in too large quantities or sizes which play a detrimental role on toughness.
Les teneurs en soufre et en phosphore de l'acier sont respectivement limitées à 0,05 et 0,1 % pour éviter une réduction de la ductilité ou de la ténacité des pièces ou des tôles fabriquées selon l'invention.  The sulfur and phosphorus contents of the steel are respectively limited to 0.05 and 0.1% in order to avoid a reduction in the ductility or toughness of the parts or sheets produced according to the invention.
L'acier peut contenir optionnellement du niobium et/ou du titane, ce qui permet d'affiner un affinement supplémentaire du grain. En raison du durcissement à chaud que ces additions confèrent, celles-ci doivent être cependant limitées à 0,050% pour le niobium et comprises entre 0,01 et 0,1% pour le titane de façon ne pas augmenter les efforts lors du laminage à chaud. A titre optionnel, l'acier peut également contenir du bore : en effet, la déformation importante de l'austénite peut accélérer la transformation en ferrite au refroidissement, phénomène qu'il convient d'éviter. Une addition de bore, en quantité comprise entre 0,0005 et 0,005% en poids permet de se prémunir d'une transformation ferritique précoce. The steel may optionally contain niobium and / or titanium, which makes it possible to refine further refinement of the grain. Due to the heat curing these additions confer, they must however be limited to 0.050% for niobium and between 0.01 and 0.1% for titanium so as not to increase the forces during hot rolling. . As an option, the steel can also contain boron: indeed, the significant deformation of the austenite can accelerate the conversion to ferrite on cooling, a phenomenon that should be avoided. Addition of boron in an amount of between 0.0005 and 0.005% by weight makes it possible to guard against early ferritic transformation.
A titre optionnel, l'acier peut également contenir du calcium en quantité comprise entre 0,0005 et 0,005% : en se combinant avec l'oxygène et le soufre, le calcium permet d'éviter la formation d'inclusions de grande taille, néfastes pour la ductilité des tôles ou des pièces ainsi fabriquées.  As an option, the steel can also contain calcium in an amount between 0.0005 and 0.005%: by combining with oxygen and sulfur, calcium prevents the formation of large inclusions, harmful for the ductility of the sheets or parts thus manufactured.
Le reste de la composition de l'acier est constitué de fer et d'impuretés inévitables résultant de l'élaboration. The rest of the composition of the steel consists of iron and unavoidable impurities resulting from the elaboration.
Les tôles ou les pièces d'acier fabriquées selon l'invention sont caractérisées par une structure totalement martensitique en lattes d'une grande finesse : en raison du cycle thermomécanique et de la composition spécifiques, la taille moyenne des lattes martensitiques est inférieure à 1 micromètre et leur facteur d'allongement moyen est compris entre 2 et 5. Ces caractéristiques microstructurales sont déterminées par exemple en observant la microstructure par microscopie électronique à balayage au moyen d'un canon à effet de champ (technique « MEB-FEG ») à un grandissement supérieur à 1200x, couplé à un détecteur EBSD (« Electron Backscatter Diffraction »). On définit que deux lattes contigi.es sont distinctes lorsque leur désorientation est supérieure à 5 degrés. La taille moyenne de lattes est définie par la méthode des intercepts connue en elle-même : on évalue la taille moyenne des lattes interceptées par des lignes définies de façon aléatoire par rapport à la microstructure. La mesure est réalisée sur au moins 1000 lattes martensitiques de façon à obtenir une valeur moyenne représentative. La morphologie des lattes individualisées est déterminée par analyse d'images au moyen de logiciels connus en eux-mêmes : on détermine la dimension maximale lmax et minimale \mm de chaque latte martensitique et son facteur d'allongement ^max . Afin d'être statistiquement représentative, cette The sheets or steel parts manufactured according to the invention are characterized by a totally slab martensite structure of great fineness: due to the specific thermomechanical cycle and composition, the average size of the martensitic slats is less than 1 micrometer and their average elongation factor is between 2 and 5. These microstructural characteristics are determined for example by observing the microstructure by scanning electron microscopy using a field effect gun ("MEB-FEG" technique) at a magnification higher than 1200x, coupled to an EBSD detector ("Electron Backscatter Diffraction"). It is defined that two contiguous slats are distinct when their disorientation is greater than 5 degrees. The average slat size is defined by the intercepts method known per se: the mean size of the intercepted slats is evaluated by randomly defined lines with respect to the microstructure. The measurement is performed on at least 1000 martensitic slats in order to obtain a representative average value. The morphology of individualized slats is determined by image analysis using software known in themselves: the maximum dimension L max and minimum \ mm is determined for each lath martensite and? Max elongation factor. In order to be statistically representative, this
/ min  / min
observation porte sur au moins 1000 lattes martensitiques. Le facteur d'allongement moyen est ensuite déterminé pour l'ensemble de cesobservation covers at least 1000 martensitic slats. The postman mean elongation is then determined for all of these
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lattes observées. slats observed.
Le procédé selon l'invention permet de fabriquer soit des tôles laminées, soit des pièces embouties à chaud ou mises en forme à chaud. Ces deux modes vont être successivement exposés.  The method according to the invention makes it possible to manufacture either rolled sheets or hot-stamped or heat-formed parts. These two modes will be successively exposed.
Le procédé de fabrication de tôles laminées à chaud selon l'invention comporte les étapes suivantes :  The process for manufacturing hot-rolled sheets according to the invention comprises the following steps:
On approvisionne tout d'abord un demi-produit d'acier dont la composition a été exposée ci-dessus. Ce demi-produit peut se présenter par exemple sous forme de brame issue de coulée continue, de brame mince ou de lingot. A titre d'exemple indicatif, une brame de coulée continue a une épaisseur de l'ordre de 200mm, une brame mince une épaisseur de l'ordre de 50-80mm. On réchauffe ce demi-produit à une température Ti comprise entre 1050°C et 1250°C. La température ΤΊ est supérieure à Ac3, température de transformation totale en austénite au chauffage. Ce réchauffage permet donc d'obtenir une austénitisation complète de l'acier ainsi que la dissolution d'éventuels carbonitrures de niobium existant dans le demi-produit. Cette étape de réchauffage permet également de réaliser les différentes opérations ultérieures de laminage à chaud qui vont être présentées : on effectue un laminage, dit de dégrossissage, du demi-produit à une température T2 comprise entre 1000 et 880°C. First, a semi-finished steel product, the composition of which has been described above, is supplied. This semi-finished product may for example be in the form of slab from continuous casting, thin slab or ingot. As an indicative example, a continuous casting slab has a thickness of about 200 mm, a thin slab a thickness of about 50-80 mm. This semi-finished product is heated to a temperature Ti of between 1050 ° C. and 1250 ° C. The temperature ΤΊ is greater than A c3 , the total conversion temperature to austenite at heating. This reheating thus makes it possible to obtain a complete austenitization of the steel as well as the dissolution of any possible niobium carbonitrides in the semi-finished product. This reheating step also makes it possible to carry out the various subsequent hot rolling operations that will be presented: a roughing operation is carried out on the semi-finished product at a temperature T 2 of between 1000 and 880 ° C.
Le taux de réduction cumulé des différentes étapes de laminage au dégrossissage est noté ε3. Si e,a désigne l'épaisseur du demi-produit avant le laminage à chaud de dégrossissage et efa l'épaisseur de la tôle après ce e The cumulative reduction rate of the various stages of rolling at roughing is noted ε 3 . If e, a is the thickness of the semi-finished product prior to hot rough rolling and ef is the thickness of the sheet after this e
laminage, on définit le taux de réduction cumulé par ε3 = Ln— . Selon rolling, the cumulative reduction ratio is defined by ε 3 = Ln-. according to
J a l'invention, le taux de réduction cumulé ε3 lors du laminage de dégrossissage doit être supérieur à 30%. Dans ces conditions, l'austénite obtenue est totalement recristallisée avec une taille moyenne de grain inférieure à 40 micromètres, voire à 5 micromètres lorsque la déformation ε3 est supérieure à 200% et lorsque la température T2 est comprise entre 950 et 880°C. On refroidit ensuite non complètement la tôle, c'est à dire jusqu'à une température intermédiaire T3, de façon à éviter une transformation de l'austénite, à une vitesse VR supérieure à 2°C/s jusqu'à une température T3 comprise entre 600°C et 400°C, domaine de température dans lequel l'austénite est métastable, c'est à dire dans un domaine où elle ne devrait pas être présente dans des conditions d'équilibre thermodynamique. On effectue alors un laminage à chaud de finition à la température T3> le taux de réduction cumulé £b étant supérieur à 30%. Dans ces conditions, on obtient une structure austénitique déformée plastiquement dans laquelle n'intervient pas la recristallisation. On refroidit ensuite la tôle à une vitesse VR2 supérieure à la vitesse de trempe critique martensitique. According to the invention, the cumulative reduction ratio ε 3 during rough rolling must be greater than 30%. Under these conditions, the austenite obtained is completely recrystallized with an average grain size of less than 40 micrometers or even 5 micrometers when the deformation ε 3 is greater than 200% and when the temperature T 2 is between 950 and 880 ° C. . The sheet is then not completely cooled, that is to say up to intermediate temperature T3, so as to avoid transformation of the austenite, at a speed V R greater than 2 ° C / s to a temperature T 3 of between 600 ° C and 400 ° C, temperature range in which Austenite is metastable, ie in a field where it should not be present under conditions of thermodynamic equilibrium. Finishing is then carried out at the temperature T 3> the cumulative reduction ratio b being greater than 30%. Under these conditions, a plastically deformed austenitic structure is obtained in which recrystallization does not occur. The sheet is then cooled at a speed VR 2 greater than the critical martensitic quenching speed.
Bien que le procédé ci-dessus décrive la fabrication de produits plats (tôles) à partir notamment de brames, l'invention n'est pas limitée à cette géométrie et à ce type de produits, et peut être mise en œuvre pour la fabrication de produits longs, de barres, de profilés, par des étapes successives de déformation à chaud.  Although the above method describes the manufacture of flat products (sheets) from slabs in particular, the invention is not limited to this geometry and to this type of products, and can be implemented for the manufacture of long products, bars, profiles, by successive stages of hot deformation.
Le procédé de fabrication de pièces embouties ou mises en forme à chaud est le suivant :  The process for manufacturing stamped or hot-formed parts is as follows:
On approvisionne tout d'abord un flan en acier dont la composition contient en poids : 0,15% < C < 0,40%, 1 ,5%< Mn < 3%, 0,005% < Si < 2%, 0,005%< Al ≤ 0, 1 %, 1 ,8% < Cr≤ 4%, 0%< Mo ≤2%, étant entendu que 2,7%<0,5 (Mn)+(Cr)+3(Mo)<5,7%, S < 0,05%, P< 0,1 %, et optionnellement : 0%< Nb<0,050%, 0,01 %≤ Ti<0,1 %, 0,0005% < B < 0,005%, 0,0005% < Ca < 0,005%.  First, a steel blank whose composition contains by weight: 0.15% <C <0.40%, 1.5% <Mn <3%, 0.005% <Si <2%, 0.005% < Al ≤ 0, 1%, 1, 8% <Cr≤4%, 0% <Mo ≤2%, with 2.7% <0.5 (Mn) + (Cr) +3 (Mo) <5 , 7%, S <0.05%, P <0.1%, and optionally: 0% <Nb <0.050%, 0.01% ≤ Ti <0.1%, 0.0005% <B <0.005% , 0.0005% <Ca <0.005%.
Ce flan plan est obtenu par découpage d'une tôle ou d'une bobine selon une forme en rapport avec la géométrie finale de la pièce visée. Ce flan peut être non-revêtu ou optionnellement pré-revêtu. Le pré-revêtement peut être de l'aluminium ou un alliage à base d'aluminium. Dans ce dernier cas, la tôle peut être avantageusement obtenue par passage au trempé en continu dans un bain d'alliage aluminium-silicium comprenant en poids 5-1 1 % de silicium, 2 à 4% de fer, optionnellement entre 5 et 30 ppm de calcium, le reste étant de l'aluminium et des impuretés inévitables résultant de l'élaboration.  This flat blank is obtained by cutting a sheet or a coil in a form related to the final geometry of the target part. This blank may be uncoated or optionally pre-coated. The pre-coating may be aluminum or an aluminum-based alloy. In the latter case, the sheet may advantageously be obtained by continuously dipping in a bath of aluminum-silicon alloy comprising by weight 5-1 1% silicon, 2 to 4% iron, optionally between 5 and 30 ppm of calcium, the rest being aluminum and unavoidable impurities resulting from the elaboration.
Le flan peut être également pré-revêtu de zinc ou d'un alliage à base de zinc. Le pré-revêtement peut être notamment du type galvanisé au trempé en continu (« Gl ») ou galvanisé-allié (« GA ») The blank may also be pre-coated with zinc or a zinc-based alloy. The pre-coating can be in particular of the type galvanized with continuous dipping ("Gl") or galvanized-alloyed ("GA")
On chauffe le flan à une température ΤΊ comprise entre Ac3 et AC3+250°C. Dans le cas où le flan est pré-revêtu, on effectue préférentiellement le chauffage dans un four sous atmosphère ordinaire ; on assiste durant cette étape à une alliation entre l'acier et le pré-revêtement. Le revêtement formé par alliation protège l'acier sous-jacent de l'oxydation et de la décarburation et se révèle apte à une déformation ultérieure à chaud. On maintient le flan à la température ΤΊ pour assurer l'homogénéité de la température en son sein. Selon l'épaisseur du flan, comprise par exemple de 0,5 à 3 mm, la durée de maintien à la température ΤΊ varie de 30 secondes à 5 minutes. The blank is heated to a temperature ΤΊ between A c3 and A C 3 + 250 ° C. In the case where the blank is pre-coated, the heating is preferably carried out in an oven under ordinary atmosphere; during this step, an alloying between the steel and the precoat is observed. The alloyed coating protects the underlying steel from oxidation and decarburization and is suitable for subsequent hot deformation. The blank is held at temperature ΤΊ to ensure the homogeneity of the temperature within it. Depending on the thickness of the blank, for example from 0.5 to 3 mm, the holding time at the temperature ΤΊ varies from 30 seconds to 5 minutes.
Dans ces conditions, la structure de l'acier du flan est complètement austénitique. La limitation de la température à AC3+250°C a pour effet de restreindre le grossissement du grain austénitique à une taille moyenne inférieure à 40 micromètres. Lorsque la température est comprise entre Ac3 et Ac3+50°C, la taille moyenne de grain est préférentiellement inférieure à 5 micromètres. Under these conditions, the steel structure of the blank is completely austenitic. The limitation of the temperature to A C 3 + 250 ° C has the effect of restricting the magnification of the austenitic grain to an average size of less than 40 micrometers. When the temperature is between Ac3 and Ac3 + 50 ° C, the average grain size is preferably less than 5 micrometers.
- on transfère le flan ainsi chauffé au sein d'une presse d'emboutissage à chaud ou bien au sein d'un dispositif de mise en forme à chaud : ce dernier peut être par exemple un dispositif de « roll-forming » dans lequel le flan est déformé progressivement par profilage à chaud dans une série de rouleaux jusqu'à atteindre la géométrie finale de la pièce désirée. Le transfert du flan jusqu'à la presse ou jusqu'au dispositif de mise en forme doit s'effectuer suffisamment rapidement pour ne pas provoquer de transformation de l'austénite.  the blank thus heated is transferred into a hot stamping press or into a hot forming device: the latter may for example be a "roll-forming" device in which the The blank is progressively deformed by hot forming into a series of rolls until the final geometry of the desired part is reached. The transfer of the blank to the press or to the shaping device must be carried out quickly enough not to cause transformation of the austenite.
- on refroidit ensuite le flan à une vitesse VRI supérieure à 2°C/s de façon à éviter la transformation de l'austénite, jusqu'à une température T3 comprise entre 600°C et 400°C, domaine de température dans lequel l'austénite est métastable.  the blank is then cooled at a speed VRI greater than 2 ° C./s in order to avoid the transformation of the austenite, to a temperature T3 of between 600 ° C. and 400 ° C., a temperature range in which the Austenite is metastable.
Selon une variante, il est aussi possible d'inverser l'ordre de ces deux dernières étapes, c'est à dire de refroidir d'abord le flan avec une vitesse VRI supérieure à 2°C/s, puis de transférer ce flan au sein de la presse d'emboutissage ou du dispositif de mise en forme à chaud, de telle sorte que celui-ci puisse être embouti ou mis en forme à chaud de la façon qui suit. On emboutit ou on met en forme à chaud le flan à une température T3 comprise entre 400 et 600°C, cette déformation à chaud pouvant être effectuée en une seule étape ou en plusieurs étapes successives, comme dans le cas du roll-forming mentionné ci-dessus. A partir d'un flan initial plan, l'emboutissage permet d'obtenir une pièce dont la forme n'est pas développable. Quel que soit le mode de mise en forme à chaud, la déformation cumulée sc doit être supérieure à 30% de façon à obtenir une austénite déformée non recristallisée. Comme les modes de déformation peuvent varier d'un endroit à un autre en raison de la géométrie de la pièce et du mode local de sollicitation (expansion, rétreint, traction ou compression uniaxiale), on désigne par ε, la déformation équivalente définie en chaque point de la pièce par et ε2 sont les According to a variant, it is also possible to reverse the order of these last two steps, ie to first cool the blank with a speed VRI greater than 2 ° C./s, and then to transfer this blank to stamping press or hot forming device, so that it can be stamped or shaped hot as follows. The blank is stamped or shaped at a temperature T3 of between 400 and 600 ° C., this hot deformation can be carried out in a single step or in several successive steps, as in the case of the roll-forming mentioned above. -above. Starting from an initial flat blank, the stamping makes it possible to obtain a part whose shape is not developable. Whatever the hot forming mode, the cumulative deformation s c must be greater than 30% so as to obtain a deformed non-recrystallized austenite. As the modes of deformation can vary from one place to another because of the geometry of the part and the local mode of stress (expansion, shrinkage, tension or uniaxial compression), we denote by ε, the equivalent deformation defined in each point of the piece by and ε 2 are the
déformations principales cumulées sur l'ensemble des étapes de déformation à la température T3. Dans une première variante, le mode de formage à chaud est choisi de telle sorte que la condition sc >30% soit satisfaite en tout endroit de la pièce formée. main deformations cumulated on all the deformation steps at temperature T 3 . In a first variant, the hot forming mode is chosen such that the condition sc > 30% is satisfied at any point in the part formed.
Optionnellement, il est également possible de mettre en œuvre un procédé de formage à chaud où cette condition ne se trouve remplie qu'à certains endroits particuliers, correspondant aux zones les plus sollicitées des pièces où l'on souhaite obtenir des caractéristiques mécaniques particulièrement élevées. On obtient dans ces conditions une pièce dont les propriétés mécaniques sont variables, pouvant résulter à certains endroits d'une trempe martensitique simple (cas de zones éventuelles non déformées localement lors de la mise en forme à chaud) et résulter dans d'autres zones du procédé selon l'invention qui conduit à une structure martensitique avec une taille de lattes extrêmement réduite et des propriétés mécaniques accrues.  Optionally, it is also possible to implement a hot forming process where this condition is fulfilled only in certain particular places, corresponding to the most stressed areas of the rooms where it is desired to obtain particularly high mechanical characteristics. In these conditions, a part is obtained whose mechanical properties are variable, which may result in some places with a simple martensitic quench (in the case of possible zones not locally deformed during hot forming) and result in other zones of the zone. method according to the invention which leads to a martensitic structure with an extremely reduced slat size and increased mechanical properties.
Après déformation à chaud, on refroidit la pièce à une vitesse V 2 supérieure à la vitesse critique de trempe martensitique de façon à obtenir une structure totalement martensitique. Dans le cas de l'emboutissage à chaud, ce refroidissement peut être réalisé par maintien de la pièce dans l'outillage avec un contact étroit avec celui-ci. Ce refroidissement par conduction thermique peut être accéléré par refroidissement de l'outillage d'emboutissage, par exemple grâce à des canaux usinés dans l'outillage permettant la circulation d'un fluide réfrigérant. After hot deformation, the part is cooled at a speed V 2 greater than the critical speed of martensitic quenching so as to obtain a totally martensitic structure. In the case of hot stamping, this cooling can be achieved by holding the piece in the tooling with close contact with it. This cooling by thermal conduction can be accelerated by cooling the stamping tool, for example through channels machined in the tool for the circulation of a refrigerant.
Outre par la composition d'acier mis en œuvre, le procédé d'emboutissage à chaud de l'invention diffère donc du procédé usuel qui consiste à débuter l'emboutissage à chaud dès que le flan a été positionné dans la presse. Selon ce procédé usuel, la limite d'écoulement de l'acier est la plus faible à haute température et les efforts requis par la presse sont les moins élevés. Par comparaison, le procédé selon l'invention consiste à observer un temps d'attente de façon à ce que le flan atteigne un domaine de température adapté pour l'ausforming, puis à emboutir à chaud le flan à température nettement plus basse que dans le procédé usuel. Pour une épaisseur de flan donnée, l'effort d'emboutissage requis par la presse est légèrement plus élevé mais la structure finale obtenue plus fine que dans le procédé usuel conduit à des propriétés mécaniques plus importantes de limite d'élasticité, de résistance et de ductilité. Pour satisfaire un cahier des charges correspondant à un niveau de sollicitation donné, il est donc possible de diminuer l'épaisseur des flans et par là même de diminuer l'effort d'emboutissage des pièces selon l'invention. In addition to the steel composition used, the hot stamping process of the invention therefore differs from the usual method of starting hot stamping as soon as the blank has been positioned in the press. According to this conventional method, the flow limit of the steel is the lowest at high temperature and the forces required by the press are the lowest. By comparison, the method according to the invention consists in observing a waiting time so that the blank reaches a temperature range suitable for the ausforming, then hot stamping the blank at a significantly lower temperature than in the usual process. For a given blank thickness, the stamping force required by the press is slightly higher but the final structure obtained thinner than in the usual process leads to greater mechanical properties of yield strength, strength and stability. ductility. To meet a specification corresponding to a given level of stress, it is therefore possible to reduce the thickness of the blanks and thereby reduce the stamping force of the parts according to the invention.
De plus, selon le procédé d'emboutissage à chaud usuel, la déformation à chaud immédiatement après emboutissage doit être limitée, cette déformation à haute température ayant tendance à favoriser la formation de ferrite dans les zones les plus déformées, ce que l'on cherche à éviter. Le procédé selon l'invention ne comporte pas cette limitation.  In addition, according to the conventional hot stamping method, the hot deformation immediately after stamping must be limited, this high temperature deformation tending to favor the formation of ferrite in the most deformed areas, which is sought to avoid. The method according to the invention does not include this limitation.
Quelle que soit la variante du procédé selon l'invention, les tôles ou les pièces d'acier peuvent être utilisés telles quelles ou soumises à un traitement thermique de revenu, effectué à une température T4 comprise entre 150 et 600°C pendant une durée comprise entre 5 et 30 minutes. Ce traitement de revenu a pour effet d'augmenter la ductilité au prix d'une diminution de la limite d'élasticité et de la résistance. Les inventeurs ont cependant mis en évidence que le procédé selon l'invention, qui confère une résistance mécanique en traction Rm d'au moins 50 MPa plus élevée que celle obtenue après trempe conventionnelle, conservait cet avantage, même après traitement de revenu avec des températures allant de 150 à 600°C. Les caractéristiques de finesse de la microstructure sont conservées par ce traitement de revenu, la taille moyenne de lattes étant inférieure à 1 ,2 micromètre, le facteur d'allongement moyen des lattes étant compris entre 2 et 5. Whatever the variant of the process according to the invention, the sheets or the steel parts may be used as such or subjected to a heat treatment of tempering, carried out at a temperature T 4 of between 150 and 600 ° C. for a period of time. between 5 and 30 minutes. This treatment of income has the effect of increasing the ductility at the price of a decrease in yield strength and strength. The inventors have however demonstrated that the method according to the invention, which gives a tensile strength Rm of at least 50 MPa higher than that obtained after conventional quenching, retained this advantage, even after tempering with temperatures ranging from 150 to 600 ° C. The fineness characteristics of the microstructure are preserved by this treatment of income, the average size of slats being less than 1, 2 micrometer, the average elongation factor of slats being between 2 and 5.
A titre d'exemple non limitatif, les résultats suivants vont montrer les caractéristiques avantageuses conférées par l'invention.  By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.
Exemple 1 : Example 1
On a approvisionné des demi-produits d'acier dont les compositions, exprimées en teneurs pondérales (%), sont les suivantes :  Steel semi-finished products have been supplied whose compositions, expressed in contents by weight (%), are as follows:
Des demi-produits de 31 mm d'épaisseur ont été réchauffés et maintenus 30 minutes à une température T-i de 1050°C puis soumis à un laminage de dégrossissage en 5 passes à une température T2 de 910°C jusqu'à une épaisseur de 6 mm, soit un taux de réduction cumulé a de 164%. A ce stade, la structure est totalement austénitique et complètement recristallisée avec une taille moyenne de grain de 30 micromètres. Les tôles ainsi obtenues ont été ensuite refroidies à la vitesse de 25°C/s jusqu'à la température T3 de 550°C où elles ont été laminées en 5 passes avec un taux de réduction cumulé £b de 60% puis refroidies ensuite jusqu'à la température ambiante avec une vitesse de 80°C/s de façon à obtenir une microstructure complètement martensitique. Par comparaison, des tôles d'aciers de composition ci-dessus ont été chauffées et maintenues 30 minutes à 1250°C puis refroidies par trempe à l'eau de façon à obtenir une microstructure complètement martensitique (traitement de référence) 31 mm thick semi-finished products were reheated and held for 30 minutes at a Ti temperature of 1050 ° C. and then subjected to a 5-pass roughing rolling at a T 2 temperature of 910 ° C. up to a thickness of 6 mm, a cumulative reduction rate of 164%. At this stage, the structure is completely austenitic and completely recrystallized with an average grain size of 30 microns. The resultant sheets were then cooled at a rate of 25 ° C / s to the temperature T 3 of 550 ° C where they were rolled in five passes with a cumulative reduction ratio of 60% £ b then cooled up to room temperature with a speed of 80 ° C / sec so as to obtain a completely martensitic microstructure. In comparison, steel sheets of the above composition were heated and held for 30 minutes at 1250 ° C. and then cooled by quenching with water so as to obtain a completely martensitic microstructure (reference treatment).
Au moyen d'essais de traction, on a déterminé la limite d'élasticité Re, la résistance à la rupture Rm, et l'allongement total A des tôles obtenues par ces différents modes de fabrication. On a également fait figurer la valeur estimée de la résistance après trempe martensitique simple (3220%(C)+908) (MPa) ainsi que la différence ARm entre cette valeur estimée et la résistance effectivement mesurée. By means of tensile tests, the yield strength Re, the tensile strength Rm, and the total elongation A have been determined for sheets obtained by these different modes of manufacture. The estimated value of the resistance after single martensitic hardening (3220% (C) +908) (MPa) and the difference ARm between this estimated value and the resistance actually measured were also included.
On a également observé la microstructure des tôles obtenues par Microscopie Electronique à Balayage au moyen d'un canon à effet de champ (technique « MEB-FEG ») et détecteur EBSD et quantifié la taille moyenne des lattes de la structure martensitique ainsi que leur facteur d'allongementThe microstructure of the plates obtained by Scanning Electron Microscopy was also observed by means of a field effect gun ("MEB-FEG" technique) and EBSD detector and quantified the average size of the slats of the martensitic structure and their factor. extension
/ max / max
moyen . way .
/ min  / min
Les résultats de ces différentes caractérisations sont présentés ci-dessous. Les essais A1 et A2 désignent des essais réalisés sur la composition d'acier A dans deux conditions différentes, l'essai B1 a été réalisé à partir de la composition d'acier B. The results of these different characterizations are presented below. Tests A1 and A2 designate tests carried out on the composition of steel A under two different conditions, the test B1 was made from the composition of steel B.
Conditions d'essais et résultats mécaniques obtenus Valeurs soulignées : non conformes à l'invention  Test conditions and mechanical results obtained Underlined values: not in accordance with the invention
La figure 1 présente la microstructure obtenu dans le cas de l'essai A1. Par comparaison, la figure 2 présente la microstructure du même acier simplement chauffé à 1250°C, maintenu 30 minutes à cette température et trempé ensuite à l'eau (essai A2) Le procédé selon l'invention permet d'obtenir une martensite avec une taille moyenne de lattes nettement plus fine et moins allongées que dans la structure de référence. Figure 1 shows the microstructure obtained in the case of test A1. By comparison, Figure 2 shows the microstructure of the same steel simply heated to 1250 ° C, maintained for 30 minutes at this temperature and then quenched with water (A2 test) The method according to the invention makes it possible to obtain a martensite with a average size of slats much thinner and less elongated than in the reference structure.
Dans le cas de l'essai A2 (trempe martensitique simple), on observe que la valeur de la résistance estimée (1536MPa) à partir de l'expression (1 ) est voisine de celle déterminée expérimentalement (1576MPa) In the case of the A2 (simple martensitic quenching) test, it is observed that the value of the estimated resistance (1536 MPa) from expression (1) is close to that determined experimentally (1576 MPa)
Dans les essais A1 et B1 selon l'invention, les valeurs de AR sont de 353 et de 306 MPa respectivement. Le procédé selon l'invention permet donc d'obtenir des valeurs de résistance mécanique nettement supérieures à celles qui seraient obtenues par une trempe martensitique simple. Cette augmentation de résistance (353 ou 306 MPa) est équivalente à celle qui serait obtenue, d'après la relation (1) par une trempe martensitique simple appliquée à des aciers dans lesquels une addition supplémentaire de 0,11 % ou de 0,09% environ aurait été réalisée. Une telle augmentation de la teneur en carbone aurait cependant des conséquences néfastes vis-à-vis de la soudabilité et de la ténacité, alors que le procédé selon l'invention permet d'atteindre de très hautes valeurs de résistance mécanique sans ces inconvénients. In tests A1 and B1 according to the invention, the AR values are 353 and of 306 MPa respectively. The method according to the invention therefore makes it possible to obtain mechanical strength values that are clearly higher than those which would be obtained by a simple martensitic quenching. This increase in strength (353 or 306 MPa) is equivalent to that which would be obtained from equation (1) by simple martensitic quenching applied to steels in which an additional addition of 0.11% or 0.09 about% would have been achieved. Such an increase in the carbon content would, however, have adverse consequences with respect to the weldability and toughness, whereas the method according to the invention makes it possible to achieve very high values of mechanical strength without these disadvantages.
Les tôles fabriquées selon l'invention, en raison de leur teneur en carbone plus faible, présentent une bonne aptitude au soudage par les procédés usuels, en particulier au soudage par résistance par points.  The plates produced according to the invention, because of their lower carbon content, have good weldability by the usual processes, in particular spot resistance welding.
Des traitements thermiques de revenu ont été ensuite réalisés dans différentes conditions de température et de durée sur l'acier dans la condition B1 ci-dessus : pour une température allant jusqu'à 600°C et une durée allant jusqu'à 30 minutes, la taille moyenne de lattes martensitiques reste inférieure à 1 ,2 micromètre. Heat treatments were then carried out under different conditions of temperature and duration on the steel in condition B1 above: for a temperature up to 600 ° C and a duration of up to 30 minutes, the Average size of martensitic slats remains less than 1, 2 micrometer.
Exemple 2 : Example 2
On a approvisionné des flans en acier d'épaisseur 3mm de composition suivante, exprimée en teneurs pondérales (%): Steel blanks of thickness 3 mm of the following composition, expressed in terms of weight (%), were supplied:
Les flans ont été soumis à un chauffage à 000°C (soit Ac3+210°C environ) pendant 5 minutes. Ceux-ci ont été ensuite : The blanks were heated at 000 ° C (ie, Ac3 + 210 ° C) for 5 minutes. These were then:
soit refroidis à 50°C/s jusqu'à la température T3 de 525°C puis emboutis à cette température avec une déformation équivalente sc supérieure à 50%, et enfin refroidis à une vitesse supérieure à la vitesse critique de trempe martensitique (essai B2) is cooled to 50 ° C / s up to the temperature T3 of 525 ° C and then stamped at this temperature with equivalent deformation s c greater than 50%, and finally cooled to a speed greater than the critical speed of martensitic quenching (test B2)
soit refroidis à 50°C/s jusqu'à la température de 525°C, puis refroidis à une vitesse supérieure à la vitesse critique de trempe martensitique (essai B3)  cooled to 50 ° C / s to 525 ° C, then cooled to above the critical martensitic quenching rate (test B3)
Le tableau ci-dessous présente les propriétés mécaniques obtenues :  The table below shows the mechanical properties obtained:
Conditions d'essais et résultats mécaniques obtenus Valeurs soulignées : non conformes à l'invention La figure 3 présente la microstructure obtenue dans la condition B3 selon l'invention, caractérisée par une taille moyenne de lattes très fine (0,9 micromètre) et un faible facteur d'allongement.  Test conditions and mechanical results obtained Underlined values: not in accordance with the invention FIG. 3 shows the microstructure obtained in the B3 condition according to the invention, characterized by a very fine slat size (0.9 micrometres) and a low elongation factor.
Ainsi, l'invention permet la fabrication de tôles, ou de pièces nues ou revêtues, à très hautes caractéristiques mécaniques, dans des conditions économiques très satisfaisantes.  Thus, the invention allows the manufacture of sheets, or bare or coated parts, with very high mechanical characteristics, under very satisfactory economic conditions.
Ces tôles ou ces pièces seront utilisées avec profit pour la fabrication de pièces de sécurité, et notamment de pièces anti-intrusion ou de soubassement, de barres de renforcement, de pieds milieu, pour la construction de véhicules automobiles.  These sheets or parts will be used profitably for the manufacture of safety parts, including anti-intrusion parts or underbody, reinforcement bars, middle feet, for the construction of motor vehicles.

Claims

REVENDICATIONS
1. Procédé de fabrication d'une tôle d'acier à structure totalement martensitique, présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen desdites lattes étant compris entre 2 et 5, étant entendu que le facteur d'allongement d'une latte de mâx 1. A method of manufacturing a steel sheet with a totally martensitic structure, having an average slat size of less than 1 micrometer, the average elongation factor of said slats being between 2 and 5, it being understood that the factor of lengthening of a latte de mâx
dimension maximale lmax et minimale n est défini par , à limite maximum dimension l max and minimum n is defined by, at limit
/min  / min
d'élasticité supérieure à 1300 MPa, à résistance mécanique supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral dudit acier, comprenant les étapes successives et dans cet ordre selon lesquelles : of elasticity greater than 1300 MPa, with a mechanical strength greater than (3220 (C) +958) megapascals, it being understood that (C) denotes the carbon content by weight percentage of said steel, comprising the successive steps and in this order according to which :
- on approvisionne un demi-produit d'acier dont la composition comprend, les teneurs étant exprimées en poids,  - supplying a semi-finished steel product whose composition comprises, the contents being expressed by weight,
0,15% < C < 0,40%  0.15% <C <0.40%
1 ,5%< Mn≤3%  1, 5% <Mn≤3%
0,005% < Si≤ 2%  0.005% <Si≤ 2%
0,005%< Al < 0,1%,  0.005% <Al <0.1%,
1 ,8% < Cr< 4%  1, 8% <Cr <4%
0%< Mo <2%  0% <Mo <2%
étant entendu que  Being heard that
2,7%<0,5 (Mn)+(Cr)+3(Mo)<5,7%  2.7% <0.5 (Mn) + (Cr) +3 (Mo) <5.7%
S < 0,05%  S <0.05%
P< 0,1%,  P <0.1%,
et optionnellement:  and optionally:
0%< Nb<0,050%  0% <Nb <0.050%
0,01 %< Ti<0,1%  0.01% <Ti <0.1%
0,0005% < B < 0,005%,  0.0005% <B <0.005%,
0,0005%≤ Ca < 0,005%,  0.0005% ≤ Ca <0.005%,
le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration, - on réchauffe ledit demi-produit à une température Ti comprise entre 1050°C et 1250°C, puis the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation, said half-product is heated to a temperature Ti of between 1050 ° C. and 1250 ° C., and then
- on effectue un laminage de dégrossissage dudit demi-produit réchauffé, à une température T2 comprise entre 1000 et 880°C, avec un taux de réduction za cumulé supérieur à 30% de façon à obtenir une tôle avec une structure austénitique complètement recristallisée de taille moyenne de grain inférieure à 40 micromètres et préférentiellement à 5 micromètres, étant entendu que ledit taux de réduction cumulé ε3 est défini par : Ln— - A rough rolling of said heated half-product is carried out at a temperature T 2 of between 1000 and 880 ° C., with a reduction ratio z a cumulated greater than 30% so as to obtain a sheet with a completely recrystallized austenitic structure. of average grain size of less than 40 microns and preferably 5 microns, it being understood that said cumulative reduction rate ε 3 is defined by:
J a eia désignant l'épaisseur du demi-produit avant ledit laminage à chaud de dégrossissage et β¾ l'épaisseur de la tôle après ledit laminage de dégrossissage, puis J ae ia designating the thickness of the semi-finished product before said hot rolling of roughing and β ¾ the thickness of the sheet after said rough rolling, and then
- on refroidit non complètement ladite tôle jusqu'à une température T3 comprise entre 600°C et 400°C dans le domaine austénitique métastable, à une vitesse VR1 supérieure à 2°C/s, puis said sheet is cooled to a temperature T 3 between 600 ° C. and 400 ° C. in the austenitic metastable domain at a speed V R1 greater than 2 ° C./s, and then
- on effectue un laminage à chaud de finition à ladite température T3, de ladite tôle non complètement refroidie, avec un taux de réduction cumulé Sb supérieur à 30% de façon à obtenir une tôle, étant entendu que ledit taux de réduction cumulé b est défini par : Ln-^— , eib désignant ef b a finishing hot rolling is carried out at said temperature T 3 , of said non-completely cooled sheet, with a cumulated reduction ratio Sb greater than 30% so as to obtain a sheet, it being understood that said cumulative reduction ratio b is defined by: Ln - ^ -, e ib designating ef b
l'épaisseur de la tôle avant ledit laminage à chaud de finition et efa l'épaisseur de la tôle après ledit laminage de finition, puis the thickness of the sheet before said finishing hot rolling and e fa the thickness of the sheet after said finishing rolling, and then
- on refroidit ladite tôle à une vitesse VR2 supérieure à la vitesse critique de trempe martensitique. said sheet is cooled to a speed VR 2 greater than the critical speed of martensitic quenching.
2. Procédé de fabrication d'une pièce d'acier à structure totalement martensitique présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen desdites lattes étant compris entre 2 et 5, étant entendu que le facteur d'allongement d'une latte de 2. A method of manufacturing a piece of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of said slats being between 2 and 5, it being understood that the elongation factor of a slat of
/ ffi tX  / ffi tX
dimension maximale lmax et minimale lmjn est défini par , comprenant maximum dimension l max and minimum l m j n is defined by, comprising
/min  / min
les étapes successives et dans cet ordre selon lesquelles : - on approvisionne un flan d'acier dont la composition comprend, les teneurs étant exprimées en poids, the successive stages and in this order according to which: a steel blank whose composition comprises, the contents being expressed by weight,
0,15% < C < 0,40%  0.15% <C <0.40%
1 ,5%< Mn < 3%  1, 5% <Mn <3%
0,005% < Si < 2%  0.005% <If <2%
0,005% AI < 0,1 %,  0.005% AI <0.1%,
1 ,8% < Cr< 4%  1, 8% <Cr <4%
0%< Mo≤2%  0% <Mo≤2%
étant entendu que  Being heard that
2,7%<0,5 (Mn)+(Cr)+3(Mo)<5,7%  2.7% <0.5 (Mn) + (Cr) +3 (Mo) <5.7%
S < 0,05%  S <0.05%
P< 0, 1 %  P <0, 1%
optionnellement:  optionally:
0%< Nb<0,050%  0% <Nb <0.050%
0,01 %< Ti<0, 1 %  0.01% <Ti <0, 1%
0,0005% < B < 0,005%,  0.0005% <B <0.005%,
0,0005% < Ca < 0,005%,  0.0005% <Ca <0.005%,
le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation,
- on chauffe ledit flan à une température Ti comprise entre Ac3 et Ac3+250°C de telle sorte que la taille moyenne de grain austénitique soit inférieure à 40 micromètres, et préférentiellement à 5 micromètres, puis said blank is heated to a temperature Ti between Ac 3 and Ac 3 + 250 ° C. so that the mean austenite grain size is less than 40 microns, and preferably 5 microns, and then
- on transfère ledit flan chauffé au sein d'une presse d'emboutissage à chaud ou d'un dispositif de mise en forme à chaud, puis said heated blank is transferred into a hot stamping press or a hot forming device, and then
- -on refroidit ledit flan jusqu'à une température T3 comprise entre 600°C et 400°C, à une vitesse VR1 supérieure à 2°C/s de façon à éviter une transformation de l'austénite, said blank is cooled to a temperature T3 of between 600 ° C. and 400 ° C., at a speed V R1 greater than 2 ° C./s in order to avoid transformation of the austenite,
- l'ordre des deux dernières étapes pouvant être interverti, puis,  the order of the last two steps that can be inverted, then
- on emboutit ou on met en forme à chaud à ladite température T3 ledit flan refroidi, d'une quantité sc supérieure à 30% dans au moins une zone, pour obtenir une pièce, étant entendu que ladite quantité c est définie par sc = , où ει et ε2 sont les déformations principales - one stamped or hot forming is brought to said temperature T 3 said cooled blank by an amount s c greater than 30% in at least one zone, to obtain a part, provided that said amount is defined by c s c =, where ει and ε 2 are the main deformations
cumulées sur l'ensemble des étapes de déformation à la température T3, puis, accumulated on all the deformation steps at the temperature T 3 , then,
- on refroidit ladite pièce à une vitesse VR2 supérieure à la vitesse critique de trempe martensitique. said part is cooled to a speed V R2 greater than the critical speed of martensitic quenching.
3. Procédé de fabrication d'une pièce selon la revendication 2, caractérisé en ce que ledit flan est embouti à chaud de façon à obtenir une pièce, puis ladite pièce est maintenue au sein de l'outillage d'emboutissage de façon à refroidir ladite pièce à une vitesse VR2 supérieure à la vitesse critique de trempe martensitique. 3. A method of manufacturing a part according to claim 2, characterized in that said blank is hot stamped so as to obtain a workpiece, then said workpiece is held within the stamping tool so as to cool said workpiece. piece at a speed V R2 greater than the critical speed martensitic quenching.
4. Procédé de fabrication d'une pièce d'acier selon l'une quelconque des revendications 2 ou 3, caractérisé en ce que ledit flan est pré-revetu d'aluminium ou d'un alliage à base d'aluminium 4. A method of manufacturing a steel piece according to any one of claims 2 or 3, characterized in that said blank is pre-coated with aluminum or an aluminum-based alloy.
5. Procédé de fabrication d'une pièce d'acier selon l'une quelconque des revendications 2 à 4, caractérisé en ce que ledit flan est pré-revetu de zinc ou d'un alliage à base de zinc 5. A method of manufacturing a steel part according to any one of claims 2 to 4, characterized in that said blank is pre-coated with zinc or a zinc-based alloy.
6. Procédé de fabrication d'une tôle ou d'une pièce d'acier selon l'une quelconque des revendications 1 à 5, caractérisée en ce qu'on soumet ladite tôle ou ladite pièce à un traitement thermique ultérieur de revenu à une température T4 comprise entre 150 et 600°C pendant une durée comprise entre 5 et 30 minutes 6. A method of manufacturing a sheet or piece of steel according to any one of claims 1 to 5, characterized in that subjecting said sheet or said piece to a subsequent heat treatment of tempering at a temperature T 4 between 150 and 600 ° C for a period of between 5 and 30 minutes
7. Tôle de limite d'élasticité supérieure à 1300 MPa d'acier, de résistance mécanique supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral dudit acier, obtenue par un procédé selon la revendication 1 , de structure totalement martensitique, présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen desdites lattes étant compris entre 2 et 5 7. Sheet of yield strength greater than 1300 MPa of steel, of mechanical strength greater than (3220 (C) +958) megapascals, with the proviso that (C) designates the percentage by weight carbon content of said steel, obtained by a method according to claim 1, having a totally martensitic structure, having an average slat size of less than 1 micrometer, the average elongation factor of said slats being between 2 and 5
8. Pièce d'acier obtenue par un procédé selon l'une quelconque des revendications 2 à 5, comportant au moins une zone de structure totalement martensitique présentant une taille moyenne de lattes inférieure à 1 micromètre, le facteur d'allongement moyen desdites lattes étant compris entre 2 et 5, la limite d'élasticité dans ladite au moins une zone étant supérieure à 1300 MPa et la résistance mécanique étant supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral dudit acier. 8. Steel part obtained by a process according to any one of claims 2 to 5, comprising at least one zone of totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of said slats being between 2 and 5, the elastic limit in said at least one zone being greater than 1300 MPa and the mechanical strength being greater than (3220 (C) +958) megapascals, it being understood that (C) denotes the carbon content in weight percent of said steel.
9 Tôle ou pièce d'acier obtenue par un procédé selon la revendication 6, de structure totalement martensitique, présentant dans au moins une zone une taille moyenne de lattes inférieure à 1 ,2 micromètre, le facteur d'allongement moyen desdites lattes étant compris entre 2 et 5 9 Sheet or piece of steel obtained by a method according to claim 6, having a totally martensitic structure, having in at least one zone an average slat size of less than 1.2 micrometres, the average elongation factor of said slats being between 2 and 5
EP12724656.9A 2011-05-12 2012-04-20 Method for the production of very-high-strength martensitic steel and sheet or part thus obtained Active EP2707513B1 (en)

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PCT/FR2012/000153 WO2012153012A1 (en) 2011-05-12 2012-04-20 Method for the production of very-high-strength martensitic steel and sheet or part thus obtained

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CA2835533C (en) 2018-12-04
RU2580578C2 (en) 2016-04-10
US20190226060A1 (en) 2019-07-25
WO2012153008A1 (en) 2012-11-15
US10895003B2 (en) 2021-01-19
JP6114261B2 (en) 2017-04-12
BR112013028931B1 (en) 2019-03-06
BR122018069395B1 (en) 2019-04-24
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UA113628C2 (en) 2017-02-27
US20140076470A1 (en) 2014-03-20
PL2707513T3 (en) 2017-04-28
US10337090B2 (en) 2019-07-02
CN103562417B (en) 2015-07-29
KR101590689B1 (en) 2016-02-01
WO2012153012A1 (en) 2012-11-15
KR20150095949A (en) 2015-08-21
RU2013155181A (en) 2015-06-20
ES2612514T3 (en) 2017-05-17
MA35058B1 (en) 2014-04-03
ZA201309348B (en) 2014-07-30
KR20140019838A (en) 2014-02-17
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